Constant thermal conductivity values of insulating materials are typically used in building design and assessment. However, the thermal conductivity depends on many factors, such as on the temperature and moisture content. Linear temperature-dependent laws have been occasionally proposed for inorganic fibrous materials such as fiberglass or rock wool that exhibit a decreased thermal conductivity (better performance) at low temperatures. However, the petrochemical-foamed insulating materials such as the polyisocyanurate, have less regular temperature dependent behaviors with a poorer performance at both extremely cold and warm temperatures. This means that the use of constant thermal conductivity values results in actual building envelope performance different from the design predictions, with increasing building energy consumptions, greater risks of condensation issues, and decreased occupants' comfort. This paper aims to quantify the impact of the temperature dependency of the thermal conductivity in exterior walls and flat roofs. Experimental results over a large temperature range (from -20 degrees C to +60 degrees C) for different insulating materials were used in hygrothermal simulations in both continental and humid temperate climates. Common insulating materials were taken into accounts, such as fiberglass, rock wool, polyisocyanurate, and extruded polystyrene. The increase in the energy fluxes from common building envelopes once the effective thermal conductivity was considered resulted below 10% for walls and as high as 70% for roofs, especially in the cold weather. Finally, a hybrid insulation system constituted by two layers of different materials, i.e. a polyisocyanurate and a rock wool is investigated. (C) 2017 Elsevier B.V. All rights reserved.